1. Field of the Invention
The present invention relates to an image heating apparatus that can be suitably used as a heat fixing device equipped in a copying machine and a printer, and more particularly to an image heating apparatus provided with a heater having a heat generation resistive member provided on a substrate and an elastic roller that forms, in cooperation with the heater, a nip portion through which a recording material that bears an image conveyed.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open No. H06-282200, a film type fixing device has been practically used as a fixing device equipped in a copying machine or a printer. The film type fixing device has a heater made of a ceramic material, a fixing film made of a polyimide or stainless steel etc. whose inner circumference is in contact with the heater and a pressure roller that forms a fixing nip portion in cooperation with the heater with the fixing film between.
In a type of film type fixing device, an elastic layer made of a silicone rubber or the like is provided on the fixing film. The elastic layer provided on the fixing film makes it possible to fix a toner image on a recording material in a surrounding manner. For this reason, this type of fixing device is mainly used in a full color printer.
In recent years, a further increase in the speed of image forming apparatuses has been demanded. To increase the speed, it is necessary to give a larger quantity of heat to the recording material in a shorter time. This requires to supply a larger electric power to the heater to increase the overall quantity of heat generated.
In the case where electric power supplied to the heater is increased with a increase in the speed of image forming apparatuses or development of color image forming apparatuses, there arises a problem that cracking of the heater can occur when a fixing device becomes uncontrollable and a large amount of power is continuously supplied due to malfunction. The fixing device is equipped with a thermosensitive element (safety element) such as a thermostatic switch that shuts down power supply to the heater when the temperature of the heater rises excessively. Therefore, no problem arises if this element works immediately when an abnormal temperature rise of the heater occurs. However, when a large amount of power is supplied to the heater, the thermosensitive element sometimes cannot respond to rapid temperature rise of the heater, and delay in operation of the thermosensitive element occurs. When the operation of the thermosensitive element delays, cracking of the heater is likely to occur.
The principal cause of cracking of the heater is mechanical shear fracture. FIG. 4 shows cracks formed in a heater 149 made of a ceramic held in a heater holder made of a resin. When a large amount of power is continuously supplied to a heat generation resistive member 150 provided on a ceramic substrate of the heater 149 as shown in FIG. 4, the temperature of the heater 149 rises excessively. As a result, the temperature of the heater support surface of the heater holder that is in contact with the heater exceeds the allowable temperature limit, so that the support surface melts.
In addition, a pressing force is applied on the heater from the pressure roller, and the heater is pushed into the heater holder together with the heater support surface. On the other hand, the temperature of the area on the ceramic substrate in which the heat generation resistive member 150 is not provided does not rise so much even when a large amount of power is continuously supplied. Accordingly, melting of the heater support surface of the heater holder does not occur in this area, and therefore the heater is not pushed into the heater holder. Consequently, a step or a difference in level occurs at the boundary between the support surface (melting surface) 151 of the heater holder that supports the area of the heater in which the heat generation resistive member 150 is provided and the support surface 152 that supports the area of the heater in which the heat generation resistive member 150 is not provided. The presence of this level difference invites concentration of stress in the heater 149. As a result, shear fracture of the heater 149 occurs. If a thermosensitive element such as a thermostatic switch operates before the heater holder is softened, cracking of the heater can be prevented. However, if the operation of the thermosensitive element is delayed as described above, cracking of the heater cannot be avoided.
When cracking of the heater occurs, the heater cannot be used any longer. This is disadvantageous from the viewpoint of recycling of parts. In addition, there is the problem that a sufficient distance cannot be left between a portion to which the primary voltage is applied via a thermistor or the like provided on the heater and the secondary circuit or the ground portion. This sometimes leads to breakage of the secondary circuit, and an additional repair cost may be incurred.